One Difficulty in Using Voluntary Transactions to Internalize Externalities Is

One difficulty in using voluntary transactions to internalize externalities is that

A) people are motivated by self-interest and are often unwilling to engage in a transaction that might make another person better off.
B) the government usually will not enforce contracts of this type.
C) transaction costs of coming to an agreement can be very large when numerous people are involved.
D) people usually don't understand what the real opportunity costs are that they face.